The performance of optical elements may be degraded by a number of mechanisms, including the following:
(1) Contamination by deposition of films or particulates or by a condensation of liquids upon optical surfaces.
(2) Surface damage such as scratches produced by the impingement of high velocity particles or by detachment of optical coatings.
These problems are especially severe in optical systems which must handle high fluxes of radiation, such as laser optics. Defects of the types mentioned above interact with the laser beam and may absorb, scatter, or focus significant amounts of energy. One or more of the following deleterious effects may result.
(1) Off-boresight flux may be increased, creating an eye safety hazard.
(2) Opaque contaminants and scratches may be subject to severe local heating, leading to fracture of optical elements.
(3) Light may be focussed by transparent droplets, causing dielectric breakdown, localized heating, etc.
(4) Light may be backscattered into the system, causing damage to the laser or other components.
High energy lasers are being applied in military, aerospace, industrial, and medical environments where laboratory standards of cleanliness and temperature control cannot be maintained. If operation of laser systems continues after the optical path has been degraded, catastrophic failure can result.
The problem is, therefore:
(1) To detect the degradation of the optical train in real time and to disable the high-energy laser before catastrophic failure occurs.
(2) To identify, if possible, the individual optical assembly or element which must be cleaned or replaced, thereby minimizing the down-time of the system and facilitating its repair by technicians.
Any of the above-mentioned degradation mechanisms will probably lead to increased levels of stray light in the optical system. The subject invention is a stray light monitor which detects the laser flux impinging on points in the optical hardware which should not be illuminated in ideal operation. When this flux exceeds a pre-set threshold, the laser is disabled.